The present invention relates to a flow sensor for measuring the mass flow rate of a fluid such as a gas flowing at a relatively small flow rate, and to a mass flow controller using the same.
Generally, to manufacture semiconductor products such as semiconductor integrated circuits, CVD and etching processes, for example, are repeatedly performed on semiconductor wafers using various semiconductor manufacturing device. In such cases, mass flow controllers are used because of the need to accurately control the feed rate of processing gas used in a small amount.
Next, the configuration of a general mass flow controller is described with reference of FIGS. 6 and 7. FIG. 6 is a schematic configuration diagram of an example of a conventional mass flow controller interposed in gas tubing, and FIG. 7 is a circuit diagram of a flow sensor of the mass flow controller.
As shown in the figures, the mass flow controller 2 is interposed in a passage of a fluid such as a liquid or gas (for example, gas tubing 4) to control the mass flowrate of the fluid. A semiconductor manufacturing device connected to one end of the gas tubing 4 is maintained, for example, at a vacuum. This mass flow controller 2 includes a flow passage 6 formed of, for example, stainless steel, and the opposite ends of the flow passage 6 are connected to the gas tubing 4. The mass flow controller 2 includes a flow sensor unit 8 disposed on the upstream side of the flow passage 6 and a flow control valve mechanism 10 disposed on the downstream side.
The flow sensor unit 8 includes a bypass assembly 12 disposed in the flow passage 6 on the upstream side of the flow of the gas fluid and produced by bundling a plurality of bypass tubes. A sensor tube 14 is connected to the opposite end sides of the bypass assembly 12 so as to make a detour around the bypass assembly 12. The sensor tube 14 allows a smaller amount of the gas fluid to flow therethrough than through the bypass assembly 12 while the ratio of the flowing amounts is held constant. More specifically, the sensor tube 14 is configured such that the ratio of the gas flowing therethrough to the total flow of the gas is always held constant. A pair of resistance wires R1 and R4 connected in series for control use are wound around the sensor tube 14, and a flow signal S1 indicating a mass flow rate is outputted through a sensor circuit 16 connected to the resistance wires R1 and R4.
The flow signal S1 is introduced into mass flow control means 18 including, for example, a microcomputer. In the mass flow control means 18, the mass flow rate of the currently flowing gas is determined based on the flow signal S1, and the flow control valve mechanism is controlled such that the determined mass flow rate becomes coincident with the mass flow rate represented by a flow setting signal Sin inputted from the outside. The flow control valve mechanism 10 includes a flow control valve 20 disposed on the downstream side of the flow passage 6. The flow control valve 20 includes a bendable diaphragm 22 made of, for example, a metal plate. Such diaphragm 22 serves as a valve element for directly controlling the mass flow rate of the gas fluid.
The flow control valve 20 is configured such that the opening degree of a valve port 24 can be optionally controlled by appropriately bending and deforming the diaphragm 22 towards the valve port 24. The upper surface of the diaphragm 22 is connected to the lower end of an actuator 26 composed of, for example, a laminated piezoelectric element, so that the opening degree of the valve can be controlled as described above. The actuator 26 is driven by a valve driving voltage S2 outputted from a valve drive circuit 28 in response to a driving signal from the mass flow control means 18.
A relationship between the resistance wires R1 and R4 and the sensor circuit 16 is shown in FIG. 7. Namely, a series connection circuit of two reference resistors R2 and R3 is connected in parallel to the series connection of the resistance wires R1 and R4, so as to form a so-called bridge circuit. A constant power supply 30 for applying a constant current is connected to the bridge circuit. In addition, a differential circuit 32 is provided, and the connection point of the resistance wires R1 and R4 and the connection point of the reference resistors R2 and R3 are connected to the input side of the differential circuit 32. The differential circuit 32 determines a potential difference between the two connection points and outputs the determined potential difference as the flow signal S1.
The resistance wires R1 and R4 are made of a material having a resistance which varies with temperature. The resistance wire R1 is wound on the upstream side of the gas flow, and the resistance wire R4 is wound on the downstream side thereof. The reference resistors R2 and R3 are kept at a substantially constant temperature.
In the mass flow controller 2 having the above configuration, when no gas fluid flows through the sensor tube 14, the bridge circuit is in equilibrium since the temperatures of the resistance wires R1 and R4 are the same. Therefore, the potential difference (the detection value of the differential circuit 32) is, for example, zero.
Assuming that the gas fluid flows through the sensor tube 14 at mass flowrate Q, then this gas fluid is heated by the heat generated by the resistance wire R1 located on the upstream side, and the heated gas fluid flow reaches the region around which the downstream side resistance wire R4 is wound. Accordingly, heat transfer occurs, and a temperature difference is generated between the resistance wires R1 and R4. In other words, a difference in resistance is generated between the resistance wires R1 and R4, and the generated potential difference is approximately proportional to the mass flow rate of the gas. Therefore, the mass flow rate of the flowing gas can be determined by multiplying the flow signal S1 by a predetermined gain. The opening degree of the flow control valve 20 is controlled using, for example, the PID control method such that the detected mass flow rate of the gas becomes coincident with the mass flow rate represented by the flow setting signal Sin (in actuality a voltage value).
As described above, the flow sensor unit 8 includes the bypass assembly 12 composed of a plurality of fine tubes and the sensor tube 14 detouring around the bypass assembly 12. In addition, the flow ratio of the diverted flow through the sensor tube 14 to the flow through the bypass assembly 12 is kept constant, and the total flow rate is measured using the flow rate detected in the sensor tube 14. Therefore, by increasing or decreasing the number of bypass tubes, the flow ratio of the diverted flow through the sensor tube 14 can be changed, whereby the flow sensor can cover a wide range extending from a small flow rate to a large flow rate. However, a flow sensor for a higher flow rate requires a larger number of fine tubes (for example, several hundred fine tubes). Disadvantageously, as the number of fine tubes increases, the size of the flow sensor will also increase, thus undesirably increasing the cost of producing the bypass assembly.
In view of the above problems, Japanese Examined Utility Model Application Publication No. Hei 3-17226 (in particular, on pages 1 to 3) has suggested a laminar flow element secured inside a sleeve. The laminar flow element includes a core concentrically inserted into the sleeve, with flat and corrugated plates being wound around the core. This laminar flow element serves as a reliable laminar flow bypass easy to manufacture at low cost. Further, Japanese Patent Application Laid-Open No. Hei 11-101673 (in particular, on pages 2 to 3 and in FIG. 4) discloses a laminar flow element including a first strip-like body having a plurality of ribs protruding from its surface and a flat second strip-like body that are wound into a tubular shape.
However, in the technology described in Japanese Examined Utility Model Application Publication No. Hei 3-17226, the inlet of a sensor pipe must be located such that an inlet length sufficient for developing a laminar flow is provided on the upstream side. As a result, it is difficult to achieve a size reduction. In addition, to change the measurable flow rate range (hereinafter referred to as flow rate range), the height and pitch of the corrugations of the corrugated plate must be changed. Therefore a different corrugated plate must be used for a different flow rate range, and this results in an increase in cost.
In Japanese Patent Application Laid-Open No. Hei 11-101673, the laminar flow element must be increased in length in order to obtain a linearity between the rate of flow through the laminar flow element and an output signal. As a result, the size of the mass flow controller increases.